Amino acid sequencing is being carried out on a scallop muscle Ca2 ions-binding protein and the two subunits of lamb kidney (Na,K)-ATPase. The results will be correlated with chemical modification studies aimed at identifying functionally important regions of the proteins. We are particularly interested in identifying the cardiac glycoside receptor site on the (Na,K)-ATPase, and in establishing the relationship of the Ca2 ion-biding protein to troponin C and other muscle regulatory proteins.